The present invention relates generally to a method for increasing energy confinement and controlling transport in the plasma of a tokamak. More particularly, the present invention relates to a method of creating and sustaining a radial electric field throughout a substantial portion of the cross-section of the plasma. The configuration and magnitude of the radial electric field that may be achieved is determined by the apparatus used to create the radial electric field and plasma and by the relationship of apparatus parameters to the resulting radial electric field.
The apparatus for toroidal magnetic confinement that is most popular in controlled fusion research today is the tokamak device. To date, the experiments that have been performed with tokamaks to create high temperature plasmas have been of short duration. The duration and energy confinement of the tokamak plasma must be increased to produce useful amounts of energy with this device. A radial electric field within the plasma of a tokamak has been experimentally shown to increase energy confinement. Theoretical work has been the basis of the proposition that radial electric fields can reduce particle and energy loss from tokamak plasmas. Experimental results reported by Oren et al, J. of Nuclear Materials 111 & 112, 34, (1982) demonstrated that overall confinement time would increase by a factor of 10 when a radial electric field was created in the plasma of a tokamak by a cold cathode or a tungsten filament. Particle and energy confinement are known to be closely related. The radial electric field that was created in the plasma by this method was of significant magnitude only at the extreme edges of the plasma. A problem which arises from longer duration plasmas is the accumulation of impurities and if fusion is occurring, fusion waste products such as helium ash. Impurity accumulation within the plasma was demonstrated by Oren et al. when a negative potential was induced within the plasma. The accumulation approached a constant value as the radial electric field decreased.
Taylor et al. Phys. Rev. Letters. 42. 446 (1979) reported that electron injection from a tungsten filament would suppress impurity influx. The injection of cold electrons was reported to reduce the sheath potential and decelerate ions before they hit the wall reducing impurity production at the wall. Control of the potential at the edge of the plasma was recognized as a mechanism for controlling production of impurities.
M. Ono et al., Phys. Rev. Letters 60, 294 (1988) reported improvements in both energy and particle confinement times associated with radial electric fields produced by radiofrequency heating of a tokamak plasma. The magnitude of internal turbulence was observed to be greatly reduced in the presence of the radial electric field, and energy confinement times improved by 30%.
Itoh and Itoh, Phys. Rev. Letters 60, 2276 (1988) and Shaing et al., Comments Plasma Phys. Controlled Fusion 12, 69 (1988) present theories which claim that radial electric fields will influence the transport of particles in tokamak plasmas and will thus affect confinement of both particles and energy. The method postulated is that a resonant interaction between the magnetic structure and particles of a particular velocity causes rapid transport of those particles to the edge of the plasma. The application of a radial electric field shifts the resonant velocity from one that many particles possess to one shared by only a few. Hence, only a few particles then participate in transport. In particular, Shaing et al. suggest that producing a negative radial electric field will reduce transport and improve confinement.
An electric field in the plasma in the presence of a magnetic field perpendicular to the electric field will cause rotation of the plasma in the plane of the electric field. A radial electric field in the plasma is perpendicular to the toroidal magnetic field, therefore a poloidal rotation of the plasma will result from a radial electric field. Poloidal plasma rotation can cause a change in plasma transport and would preferentially affect heavier ions which are generally impurities in a tokamak plasma. Poloidal plasma rotation may therefore provide an additional method for impurity control.
Radial electric fields have been shown to produce significant increases in confinement of the plasma of a tokamak and to exert a significant influence on the production of impurities. The methods and apparatus which have been used to induce radial potential gradients and electric fields have been effective only at the edge of the plasma. The ability to induce a radial electric field throughout a substantial area of the cross-section of the plasma would provide a mechanism for additional control of radial transport and the potential for further improvement in confinement and impurity controls in tokamaks.
Therefore, in view of the above, it is an object of the present invention to provide a method for creating and maintaining a radial electric field throughout a substantial cross-section of the plasma of a tokamak. It is another object of the present invention to provide a method for creating and maintaining a radial electric field of a desired configuration and magnitude.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.